The invention relates to a circuit arrangement for generating a sawtooth-shaped signal for the field deflection in a picture display device with an amplitude which is substantially independent of the field frequency, the circuit arrangement comprising a sawtooth generator which is triggerable by means of a field synchronizing signal and which has an output terminal for making the sawtooth-shaped signal available, and means for comparing the value of the sawtooth-shaped signal during the occurrence of a sampling pulse with a reference value and for applying a control signal to a control input of the sawtooth generator for controlling the slope of the generated sawtooth-shaped signal.
A circuit arrangement of this type is known from U.S. Pat. No. 4,490,653. In this known arrangement the amplitude of the generated sawtooth-shaped signal determining the height of the picture displayed on a display screen is maintained substantially constant at each value of the field frequency, for example, 50 or 60 Hz for the European or American television standard. Such a control must be fast: in fact, it is a requirement that upon switching on the picture display device the steady state of the control loop is achieved before the picture appears on the screen.
When the incoming field synchronizing signal has a phase jitter, the reaction speed of the amplitude control loop causes an instability in the picture height, which is troublesome. Such an instability occurs when a television signal is displayed which originates from a picture recording and display device for a special mode of this device, i.e. for example, at an accelerated or slow-motion display. In this case it appears that a field period is slightly too short and that the subsequent field period is slightly too long. This applies to picture recording and display devices having two magnetic heads. A similar phenomenon occurs in a teletext non-interlace 312/313 mode when using a computer controlled teletext decoder (CCT) in which the duration of the even fields is 312 line periods and the duration of the odd fields is 313 line periods instead of the interlace mode in which all fields have a duraton of 3121/2 line periods (European standard). Due to the different durations of the successive field periods the known amplitude control loop will set the picture height at a different value during each field and therefore cause an incorrect interlacing of the fields.
It is evident that the above-mentioned difficulty is caused by the high speed of the amplitude control loop. It is then obvious to decrease this speed. The interlace error is then reduced with an increasing integrated period. This implies, however, that the height of the picture slowly increases after the picture display device has been switched on, so that a distorted picture is visible for some time. This is very undesirable.